Our overall goal is to understand muscle coordination of the human lower limbs. Muscle coordination is constrained by both the mechanics of the motor task and the ability of the nervous system to generate muscle activity patterns. Our past work has focused on how the biomechanical constraints of the musculoskeletal system and its interaction with the environment impact muscle coordination. The role of computer models has been important to our studies on multijoint lower limb motor tasks (jumping, posture, walking, pedaling). These computer models, based on mechanics and physiological concepts, have been pivotal to the formalization of biomechanical principles, which must be accounted for in the neural strategy controlling the motor task. The focus in this next project period is on how the nervous system generates muscle activity patterns to control interleg coordination. Emphasis is placed on the identification of the nervous system constraints and how sensory information from the ipsilateral and contralateral legs is used. The constraints to be considered are muscle synergies (i.e., similar excitation of different muscles in the same limb) and interlimb synergy coupling (i.e., exultation of synergies in the two legs to be alternating or in-phase). Pedaling a stationary ergometer is a motor task well suited to the study of interleg multijoint muscle coordination. Our past work on pedaling biomechanics and muscle coordination suggests that the ability to pedal can be realized by grouping all the muscles in each leg into three agonist-antagonist synergy pairs, where the two synergies of each pair alternate with each other in the crank cycle and with their counterparts m the other leg. A simple conceptual model of how sensory information of position and motion is used to phase the synergies is being proposed. Backward and forward pedaling, initiation of pedaling, and servomotor- assisted one-legged pedaling (where the other, mechanically-decoupled leg is either stationary or moving, and either passive or active) will be studied to develop these concepts of muscle synergies, interlimb synergy coupling, and afferent control. The computer sensorimotor control model to be developed will provide a framework for the diagnosis and rehabilitation of persons with neurological impairments of the lower limbs.